1. Field of the Invention
The present invention relates to an optical printer with a spatial light modulator constituted of a micromirror device having a large number of very small mirrors with variable reflecting direction.
2. Background Arts
Optical printers print pictures, including characters and drawings, on photosensitive materials, such as photographic paper. Of the optical printers, color photographic printers that project an optical image of a picture frame from negative photographic film onto photographic paper have been widely used. Recently, digital optical printers have been developed and used in practice that drive a display device in accordance with digital image data to print a picture by projecting printing light beams or an optical image from the display device onto a photosensitive material. As the digital optical printers, there are CRT type, Laser type, LCD type, micromirror type and so forth.
The micromirror type optical printer uses a spatial light modulator constituted of an array of a large number of very small mirrors, called a micromirror array. The micromirror array is arranged in a row or in a matrix, i.e. in a plurality of rows. The micromirrors may tilt each individually to reflect a spot light beam in a variable direction. Exemplars of the micromirror type optical printers are disclosed in Japanese Laid-open Patent Application Nos. 9-164727 and 9-314910. Since the micromirror type optical printer utilizes reflection of mirrors, the attenuation of light is smaller than other types, so the printing light is most efficiently utilized. Of the micromirror type spatial light modulator, there are digital micromirror devices (DMD) and piezo-drive type micromirror device (AMD). The DMD uses electrostatic power to make the micromirrors incline, whereas the AMD uses very small piezoelectric elements to make the micromirrors incline.
In the optical printers, each micromirror of the DMD and AMD corresponds to one pixel. In some of the DMD and AMD, the micromirror array corresponds in pixel number and frame size to one picture frame to print. In other types, the micromirror array corresponds to a fraction of one picture frame, i.e. several lines or several tens of lines of one picture frame. Where the DMD or the AMD corresponds to one picture frame, the photosensitive material is advanced by one frame after the entire picture is printed. Where the DMD or the AMD corresponds to a fraction of one picture frame, the picture is printed one fraction after another while advancing the photosensitive material one fraction after the printing of one fraction.
In the latter type, since the photosensitive material is advanced several steps per one picture frame, if the advanced length fluctuates in one step, a gap or an overlap is provided between the preceding fraction and the next fraction. The gap or the overlap respectively appears as a white stripe or a black stripe, and deteriorates the quality of the photograph.
In view of the foregoing, an object of the present invention is to provide an optical printer, wherein a picture is printed while advancing the photosensitive material several steps per one picture frame, whereby the printed picture frame does not suffer any gap or overlap even if the advanced length of the photosensitive material fluctuates.
To achieve the above object in an optical printer that comprises a spatial light modulator having an array of micromirrors arranged in a row or in a matrix, the micromirrors being able to tilt each individually between an effective reflecting position and an ineffective reflecting position; a driving device for driving each of the micromirrors in accordance with image data to tilt either to the effective reflecting position or to the ineffective reflecting position; a light source section for illuminating the micromirrors; an image projecting optical system for projecting printing light beams from those ones of the micromirrors which are in the effective reflecting position, as an optical image of a predetermined size onto a photosensitive material; and an advancing device for advancing the photosensitive material intermittently by a length corresponding to the size of the optical image, the present invention suggests providing a detection device for detecting fluctuations in advanced length of the photosensitive material by the paper advancing device; and a correction device for correcting projecting position of the optical image on the photosensitive material on the basis of fluctuation values detected by the detection device, by displacing the spatial light modulator.
According to another aspect of the invention, projecting position of the optical image on the photosensitive material is corrected by displacing a member of the image projecting optical system on the basis of fluctuation values detected by the detection device.
According to a preferred embodiment, the correction device accumulates fluctuation values detected by the detection device, to correct the image projecting position on the photosensitive material on the basis of an accumulated fluctuation value. The correction device also controls the paper advancing device to change the length of advancement of the photosensitive material when the accumulated fluctuation value goes beyond a range within which the image projecting position is correctable by the displacement of the spatial light modulator, so as to reduce the accumulated fluctuation value to be within the range.
The correction device preferably comprises a piezoelectric element for displacing the spatial light modulator.